As a solid precursor to O2 and hydrogen peroxide (H2 O2 ), calcium peroxide (CaO2 ) has found widespread use in applications related to disinfection and contaminant degradation. The lack of uniform nanoparticles, however, greatly limits the potential use of this material in other applications related to medicine. Here, a new route to the facile synthesis of CaO2 nanocrystals and their spherical aggregates with uniform, controllable sizes is reported. The synthesis involves the reaction between CaCl2 and H2 O2 to generate CaO2 primary nanocrystals of 2-15 nm in size in ethanol, followed by their aggregation into uniform, spherical particles with the aid of poly(vinyl pyrrolidone) (PVP). The average diameter of the spherical aggregates can be easily tuned in the range of 15-100 nm by varying the concentrations of CaCl2 and/or PVP. For the spherical aggregates with a smaller size, they release H2 O2 and O2 more quickly when exposed to water, resulting in superior antimicrobial activity. This study not only demonstrates a new route to the synthesis of uniform CaO2 nanocrystals and their spherical aggregates but also offers a promising bacteriostatic agent with biodegradability.